The present invention relates to novel magnesium silicates and methods for their preparation. In particular, the present invention relates to novel magnesium silicates having a porous structure and catalytic properties.
Silicates are materials having as a fundamental unit a tetrahedral complex consisting of Si.sup.4+ in tetrahedral coordination with four oxygens. Less common but also known is an octahedral complex consisting of Si.sup.4+ in octahedral coordination with six oxygens. In some structures, the tetrahedra link to form chains or sheets. If the linking occurs in three dimensions, what is termed a framework structure results. Framework silicates may have substituted elements in place of some of the silicon atoms. A common substitution is that of aluminum for silicon forming what are known as aluminosilicates.
Zeolites are typically crystalline, hydrated aluminosilicates of Group I or Group II elements as formed in nature or synthesized. Structurally the zeolites are porous crystalline framework aluminosilicates which are based on an extending three-dimensional network work of SiO.sub.4 and AlO.sub.4 tetrahedra linked to each other by shared oxygens.
Molecular sieves are compositions which have the property of acting as sieves on a molecular scale. Dehydrated crystalline zeolites are important molecular sieves. Zeolites, as well as coals, oxides, glasses, intercalation compounds of graphite and alkali metals, pillared clays, and carbon absorbents are types of molecular sieves. Classification and identification of the aforementioned compositions is difficult with varied and inconsistent usage of terms found in the literature. For further information and background regarding molecular sieves and zeolites see Breck, Zeolite Molecular Sieves, John Wiley & Sons (1974).
Molecular sieves in general and zeolite catalysts in particular have found widespread industrial use. From a catalytic viewpoint, zeolites are molecular sieves having molecular cages with variable dimensions. Zeolite catalysts have the important properties of (1) ingress/egress control of molecules, (2) high density of "active" sites, (3) sites for occluded or deposited chemical moieties such as salts or metals, and (4) controllable potential energy fields. Zeolites are useful as catalysts in such processes as catalytic cracking, conversion of methanol to gasoline, polymerization reactions, and alkylating aromatics to mention a few. Also, molecular sieves find such utilities as acting as selective absorbents or facilitating separation of isomers.
Zeolite molecular sieves are found in nature and have been known for many years. The most siliceous natural zeolites are ferrierite, clinoptilolite and mordenite all having a SiO.sub.2 /Al.sub.2 O.sub.3 ratio of about 10:1. Synthetic zeolites were first commercially produced with the introduction of zeolite A around 1954. The first synthetic zeolites had SiO.sub.2 /Al.sub.2 O.sub.3 ratios in the range of 2:1 to 10:1. Later synthetic zeolites were made having molar ratios in excess of 10:1.
In 1972, U.S. Pat. No. 3,702,886 (Argauer) issued for a synthetic zeolite termed ZSM-5 and method for making same. This patent disclosed a zeolite having a SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio from about 5 to 100. The main claim characterized ZSM-5 by reference to a table of X-ray diffraction lines (see Table I infra) and the following composition in terms of mole ratios of oxides EQU 0.9.+-.0.2M.sub.2/n O:Al.sub.2 O.sub.3 :YSiO.sub.2 :ZH.sub.2 O
wherein M is at least one cation having a valence n, Y is at least 5 and Z is between 0 and 40.
TABLE I ______________________________________ ZSM-5, Interplanar Spacing d(A) ______________________________________ 11.1 .+-. 0.2 6.30 .+-. 0.1 5.01 .+-. 0.1 3.71 .+-. 0.05 10.0 .+-. 0.2 6.04 .+-. 0.1 4.60 .+-. 0.08 3.04 .+-. 0.03 7.4 .+-. 0.15 5.97 .+-. 0.1 4.25 .+-. 0.08 2.99 .+-. 0.02 7.1 .+-. 0.15 5.56 .+-. 0.1 3.85 .+-. 0.07 2.94 .+-. 0.02 ______________________________________
The ZSM-5 aluminosilicate is prepared by including nitrogenous organic molecules such as tetrapropyl ammonium bromide in the reaction mixtures. For very high SiO.sub.2 /Al.sub.2 O.sub.3 preparations, no aluminum need be deliberately added since it is present as an impurity in the reactants. The organic molecules are incorporated into the framework structure as it forms and these as-synthesized materials are termed "nitrogenous zeolites". Application of high temperatures will free high SiO.sub.2 /-Al.sub.2 O.sub.3 materials of these organic components without altering the basic framework structure, D. M. Olson et al., "Chemical and Physical Properties of the ZSM-5 Substitutional Series", J. Catal., 61, 390-396 at 391 (1980).
It is known that many properties of zeolites are primarily dependent upon the framework structure while remaining essentially invariant with composition changes such as altering the SiO.sub.2 /Al.sub.2 O.sub.3 ratio. These properties include the X-ray diffraction pattern, pore size and volume, framework density and refractive index. Other properties of zeolites such as ZSM-5 will vary with composition. These properties include catalytic activity, ion-exchange capacity, and hydrophobicity (see Olson, supra, at 391).
A process for alteration of the SiO.sub.2 /Al.sub.2 O.sub.3 ratio as well as zeolite composition in general was disclosed in 1973 in U.S. Pat. No. 3,761,396 (Pickert II). This patent reviews means for removing aluminum from aluminosilicates to produce more siliceous materials. This patent also reports that in some instances, aluminum in the zeolitic crystalline structure can be substituted by other metals. In a related earlier patent, U.S. Pat. No. 3,640,681 (Pickert I) which issued in 1972, the extraction of framework aluminum from large pore crystalline zeolitic molecular sieves is examined in detail. A process is claimed which utilizes acetylacetonate as an agent for aluminum removal. Also when used with a metal acetylacetonate, the metal can be substituted in the framework for the aluminum. Examples of such substitution are given which incorporate vanadium and chromium into Y zeolites. Metal acetylacetonates are preferred which utilize metals that form oxides in tri-, tetra- or pentavalent states and which are thermally stable at 600.degree. C. However, mono- and divalent metals can also be used according to the written description which lists Mg.sup.++ as being among those metals suitable for substitution.
U.S. Pat. No. 4,049,573 (Kaeding) issued in 1977 disclosed a class of zeolites exemplified by ZSM-5, ZSM-11, ZSM-12, ZSM-35, and ZSM-38 whose members may be treated by impregnation with magnesium, boron or phosphorus compounds. These zeolites are catalysts having occluded or deposited metal moieties. These modified zeolites are useful as catalysts in hydrocarbon conversion reactions.
U.S. Pat. No. 4,061,724 (Grose) issued in 1977 for a crystalline silica composition known as "silicalite" and method for preparing same. This composition has many molecular sieve properties similar to porous crystalline aluminosilicates, but does not have ion-exchange properties which are essential to zeolitic molecular sieves. Furthermore, the silicalite does not contain AlO.sub.4 tetrahedra as essential framework constituents. Silicalite has an X-ray diffraction pattern very similar to the pattern for ZSM-5.
A detailed comparison of silicalite and ZSM-5 which discloses that both silicates are members of the same structural family is presented in Olson, D. H. et al., "Chemical and Physical Properties of the ZSM-5 Substitutional Series", J. Catal., 61, 390-396 (1980).
U.S. Pat. No. 4,108,881 (Rollman I) discloses an aluminosilicate zeolite termed ZSM-11. The structure of ZSM-11 has been related to that of ZSM-5 and to that of silicalite in an article by G. T. Kokotailo and W. M. Meier, "Pentasil Family of High Silica Crystalline Materials", Spec. Publ.-Chem. Soc., 33 (Prop. Appl. Zeolites), 133-139 (1980). This article describes the common structural features of the above materials and proposes a generic name--pentasil--to denominate this family of structures.
U.S. Pat. No. 4,148,713 (Rollman II) discloses ZSM-5 particles having aluminum-free outer shells of SiO.sub.2.
Within the group of framework silicates with tridimensional structure there is a plethora of examples which illustrate substitution of Si.sup.4+ by metal ions other than Al.sup.3+, as described by W. Eitel in the well-known book collection Silicate Science, Academic Press, 1964.
The substitution of Si.sup.4+ by Al.sup.3+ is common in framework silicates, but other metal ions are frequently found as substituents as well. For example, iron substitution is widespread in natural zeolites and in natural framework silicates.
Less common is the substitution of Si.sup.4+ by divalent ions. In an article by H. Y-P Hong, "New Solid Electrolytes", Adv. Chem. Ser., 163, 179-194, A.C.S. 1977, the substitution of Si.sup.4+ by divalent ions including magnesium in framework silicates is described. H. Y-P Hong distinguishes the properties of the resulting silicates in terms of the specific cations required to balance the electronic charge of the framework.
Recently publications have issued disclosing materials resembling zeolites in crystalline structure, but not being aluminosilicates. The framework atoms of these typically pentasil structured materials contain combinations of elements other than the usual list of aluminum, silicon, oxygen, gallium or germanium.
In European Patent Office document No. 3,622 (Maas et al.) published in January of 1979, the applicant, Shell International Research, discloses a process for the preparation and separation of organic isomers using crystalline silicates which contain iron atoms within the framework structure. These silicates have a pentasil structure similar to that of ZSM-5.
In United Kingdom Patent application Nos. 2,023,562A (Taramasso I) published Jan. 3, 1980 and 2,024,790A (Taramasso II) published Jan. 16, 1980, applicant, Snamprogetti S.p.A., discloses materials giving X-ray diffraction patterns akin to ZSM-5. In the process of forming these materials, aminoalcohols, such as triethanolamine may be used. The materials formed may be used as catalysts. In addition to containing oxygen and silicon as part of the framework structure, these silicates may also include boron, beryllium, chromium, zinc, titanium, vanadium, manganese, iron, cobalt, zirconium, rhodium, silver or antimony.
In German patent document No. 2830787 (Marosi) published Jan. 23, 1980, applicant, BASF, discloses a method for the production of nitrogen-containing crystalline metal silicates with zeolite structure. This document also discusses the framework substitution of metals in place of aluminum. Boron, arsenic, antimony, vanadium, iron or chromium are given as examples of metals having a valence of three which may be substituted into the framework structure. It also states that elements such as chromium, vanadium and arsenic may be partially framework-substituted and partially deposited in the intracrystalline pores. These new materials are pentasil structured having a ZSM-5 or ZSM-8 structure in particular. A method of making this material is disclosed which utilizes hexamethylenediamine.
In United Kingdom patent document No. 2,033,358A (Morrison), published May 21, 1980, applicant, Mobil, discloses crystalline materials having ZSM-5 zeolite structures whose composition has silica to aluminum ratios ranging from about 35 to about 3000 or more. The composition of these materials may include rare earth metals, chromium, vanadium, molybdenum, indium, boron, mercury, tellurium, silver, ruthenium, platinum or palladium, however, it is unclear how these elements are related in the composition.
In European Patent Office document No. 13,630 (Rosinski), published July 23, 1980, applicant, Mobil, discloses a ZSM-12 type material which contains chromium or iron or both with the chromium and iron believed to be in positions of tetrahedral-substitution within the silica lattice.
Mobil has also disclosed in European Patent Office document No. 14,545 (Chu et al.), published Jan. 24, 1980, that the ZSM-5 class of zeolite catalysts are shape selective and that this shape selectivity can be enhanced by impregnation with magnesium or phosphorus to reduce zeolite pore openings as well as by the use of very large crystals and coke selectivation. This document further discloses impregnating or ion-exchanging a zeolite with metal-containing salts such as Mg, followed by high temperature calcination at about 649.degree. C. or higher. In the examples, a solid-state exothermic reaction of magnesium impregnated ZSM-5 is reported at 871.degree. C. with the reaction believed to be between magnesium and ZSM-5.
U.S. Pat No. 4,229,424 (Kokotailo), discloses a crystalline zeolite product having a structure intermediate of that of ESM-5 and ZSM-11.